Trypanosome variable surface glycoproteins: composite genes and order of expression

Combinatorial processes increase the diversity of variable surface glycoproteins (VSGs) expressed by Trypanosoma equiperdum. We show here that a single telomeric pseudogene provides the 3' portion of three distinct T. equiperdum VSG genes by recombination with different 5' donor pseudogenes. Regions of sequence homology among the pseudogenes determine the sites of recombination in the formation of the expressed copies. This suggests that the recombination between any given basic copy (BC) and the expression-linked copy (ELC) depends on their sharing homology. We present evidence that this is the case and propose that such rules account for the order of expression of the VSGs. These results demonstrate how homologous recombination can generate an ordered sequence of gene expression.